Membrane structure and method of making

ABSTRACT

A membrane structure is provided. The membrane structure includes a first layer having a plurality of interconnected pores; and a second layer disposed on the first layer. The second layer has a plurality of unconnected pores. Each of the unconnected pores is in fluid communication with at least one of the interconnected pores of the first layer. A method of making a membrane structure is provided. The method includes the steps of providing a first layer having a plurality of interconnected pores; and disposing a second layer on the first layer. Disposing a second layer includes depositing a conducting layer on the first layer; and anodizing the conducting layer to convert the conducting layer into a porous layer.

BACKGROUND

The invention relates generally to a membrane structure. Moreparticularly, the invention relates to a membrane structure havingsubstantially high flux and substantially high selectivity. Theinvention also relates to a method of making a membrane structure.

Porous membrane structures have been extensively used in filtration,separation, catalysis, detection, and sensor applications. Realizingmembrane structures with fine pores and high flux is difficult, as theflux through the membrane decreases with decreasing pore size.Therefore, typically layers with fine pores are made very thin.Fabricating thin porous layers with uniform pores over large surfacearea and which are mechanically robust is a challenging task. Therefore,typically thin fine porous membranes are stacked on thicker substrateswith coarser pores. In such membrane structures it is extremelydifficult to get a defect free smooth interface between layers to ensurefaultless connectivity through the membrane structure. In spite of mucheffort, the currently available membrane structures with fine poresexhibit undesirably low permeance. Therefore, it is desirable to improvethe efficiency of fine porous membrane structures suitable for hightemperature, high pressure, and/or corrosive atmospheres.

SUMMARY OF THE INVENTION

The present invention meets these and other needs by providing amembrane structure, which has high flux and high selectivity.

Accordingly, one aspect of the invention is to provide a membranestructure. The membrane structure includes a first layer having aplurality of interconnected pores; and a second layer disposed on thefirst layer. The second layer has a plurality of unconnected pores. Eachof the unconnected pores is in fluid communication with at least one ofthe interconnected pores of the first layer.

A second aspect of the invention is to provide a method of making amembrane structure. The method includes the steps of providing a firstlayer having a plurality of interconnected pores; and disposing a secondlayer on the first layer. Disposing a second layer includes depositing aconducting layer on the first layer; and anodizing the conducting layerto convert the conducting layer into a porous layer.

BRIEF DESCRIPTION OF DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a schematic representation of a first-order branched porestructure, according to one embodiment of the present invention;

FIG. 2 is a schematic representation of a membrane structure, accordingto one embodiment of the present invention;

FIG. 3 is a schematic representation of a membrane structure, accordingto another embodiment of the present invention;

FIG. 4 is a schematic representation of a gas separation assemblyincorporating membrane structure of the invention, according to oneembodiment of the invention;

FIG. 5 is a schematic representation of a filter incorporating membranestructure of the invention, according to one embodiment of theinvention;

FIG. 6 is a flow chart of a method of making membrane structure,according to one embodiment of invention;

FIG. 7 is a schematic representation of a method of making membranestructure, according to one embodiment of invention; and

FIG. 8 is a schematic representation of a method of making membranestructure, according to another embodiment of invention.

DETAILED DESCRIPTION

In the following description, like reference characters designate likeor corresponding parts throughout the several views shown in thefigures. It is also understood that terms such as “top,” “bottom,”“outward,” “inward,” “first,” “second,” and the like are words ofconvenience and are not to be construed as limiting terms. Furthermore,whenever a particular aspect of the invention is said to comprise orconsist of at least one of a number of elements of a group andcombinations thereof, it is understood that the aspect may comprise orconsist of any of the elements of the group, either individually or incombination with any of the other elements of that group.

Referring to the drawings in general, it will be understood that theillustrations are for the purpose of describing one embodiment of theinvention and are not intended to limit the invention thereto.

For the purposes of understanding the invention, the term “layer withsubstantially unconnected pores” is to be understood to be a porouslayer in which pore connections are limited to, at most, a first-orderbranched structure 2, as illustrated in FIG. 1. A “first-order branchedstructure” as that term is used herein comprises one or more main stempores 4 from which one or more branch pores 6 radiate, where a branchpore has no junctions with any other pore aside from its junction withits main stem pore. Note that the term “layer with substantiallyunconnected pores” includes, in addition to first-order branchedstructures, structures with completely isolated pores, as in a structureconsisting of parallel, unconnected channels. Of course, one skilled inthe art will recognize that an occasional defect is to be expected infabricating such structures, and so a structure containing occasionaldefects (such as, for example, a small number of branch pores which jointo other branch pores in violation of the above definition, or a smallamount of connectivity between otherwise parallel, unconnected porechannels) will still be considered a structure with substantiallyunconnected pores if the number of defects is not sufficient tosubstantially alter the performance of the structure relative to whatwould be expected for a defect-free structure.

Schematic representation of a membrane structure according to oneembodiment of the present invention is shown in FIG. 2. The membranestructure 10 of FIG. 2 includes a first layer 12 having a plurality ofinterconnected pores 14 and a second layer 16 having a plurality ofunconnected pores 18 disposed on the first layer 12. Typically, each ofthe unconnected pores of the second layer 14 is in fluid communicationwith at least one of the interconnected pores of the first layer 12. Thedistinct connection between the layers establishes a registry betweenlayers of the membrane structure and ensures high flux. In someconventional membrane structures, the porous layers are sandwichedtogether to form a membrane structure. In such structures, theestablishment of fluid communication among pores of different layers isaccomplished by probability—the chances that pores will align in thestructure as one layer is coated onto or otherwise applied to the otherlayer. This dependence on probability to establish pore alignmentresults in a large number of misaligned pores that end up isolated from,or substantially occluded from, the rest of the structure. Any suchdefect or other irregularity at the interface may reduce the fluxthrough the membrane structure. Therefore, achieving a faultlessconnectivity between the layers is critical to a membrane's performancefor separator or filter applications. The membrane structures of thepresent invention have been designed to substantially mitigate suchproblems.

Typically the first layer includes a layer with a plurality ofinterconnected pores. The first layer may be a porous ceramic or aporous polymer layer. In one embodiment, the first layer has a porosityvolume fraction of at least about 1%. In another embodiment, the firstlayer has a porosity volume fraction in the range from about 20% toabout 70%. In yet another embodiment, the membrane structure has aporosity volume fraction in the range from about 30% to about 50%.

The total thickness of the membrane structure is chosen in such a waythat the structure is thick enough for mechanical robustness, but not sothick as to impair permeability. The thickness of the individual layersis optimized depending on the end-use application. In one embodiment,the second layer has a thickness less than about 10 micrometers. Inanother embodiment, the second layer has a thickness in the range fromabout 10 nanometers to about 500 nanometers. In another embodiment, theinner layer has a thickness in the range from about 10 nanometers toabout 100 nanometers.

Precise control over pore size and pore size distribution are among theparameters that define the membrane performance. The pore size of thelayers is chosen based on the end use application of the membranestructure. In some embodiments, the second layer has a median pore sizeof less than about 1 micrometer. In other embodiments, the median poresize of the second layer is in a range from about 1 nanometer to about500 nanometers. In some other embodiments, the median pore size of thesecond layer is in the range from about 1 nanometer to about 40nanometers. In these embodiments, the pore size of the first layer ischosen so that they do not hinder the permeance of the species throughthe membrane structure.

In an exemplary embodiment, the second layer includes a plurality ofcylindrical pores of uniform size all aligned approximatelyperpendicular to the membrane surface. Straight pores with lowtortuosity reduce fluid dispersion and facilitate high flux through themembrane structure.

The material of the first layer and the second layers are chosen basedon the end use application. Typically the first layer includes either apolymer or a ceramic with a suitable porosity, pore dimensions, andthickness. In an exemplary embodiment, the first layer includes aceramic. Non-limiting examples of ceramics are oxides, carbides,nitrides, borides, and silicides. Examples of suitable ceramics include,but are not limited to, aluminum oxide, silica, silicate, rare-earthoxide, titania, zirconia, lanthana, yttria stabilized zirconia, aperovskite, a spinel, vanadia, ceria, and combinations thereof. In someembodiments, the ceramic may include a suitable dopant. Ceramicmaterials have the advantages of thermal and chemical stability, gooderosion resistance, and high-pressure stability. Thus the membranestructures of the invention may withstand prolonged exposure to pressureor temperature differences that may be present in, for example, a gasseparation or sensor assembly.

In some embodiments, the first layer includes a polymer. Suitablepolymers that may be used include, but are not limited to,polysulphones, polyamides, cross-linked polyimides, polyether ketones,polyetherimides, silicone rubber, nitrile rubber, neoprene rubber,silicone, polycarbonate, polyarylene, polyphenylene ether, polyolefinelastomer, polybutadiene, vinyl polymers, or other thermoplasticpolymers, combinations thereof, and block copolymers of these. Thesepolymers may be used to achieve specific functionalities. For example,silicone rubber is very effective in removing volatile organiccomponents such as toluene, methanol, methylene chloride, and acetonefrom gas streams.

In certain embodiments, the first layer includes more than one sublayer.In such embodiments, the sublayer not in contact with the second layermay include a metal. A pure metal or a metal alloy may be used. Themetal may be applied on the membrane layers as a dispersed particulate,or a continuous coating, or a metal layer may be inserted into themembrane structure. In some embodiments, the membrane pore walls may becoated with a metal. The metal may be disposed into the membranestructure by any known coating technique including exposing thestructure to a suspension of metal particulates, by electrolessdeposition, or electroplating, or chemical vapor deposition or physicalvapor deposition techniques. In some embodiments, the metal is aplatinum group metal. In one embodiment palladium with copper, gold orsilver is used. In another embodiment, an alloy of palladium withruthenium, osmium, nickel, platinum, or a combination of these is used.In some embodiments, the transition metal elements such as iron, nickel,cobalt, or copper may be included in the membrane structure. Manytransition metal complexes show selective interaction with molecularoxygen involving reversible chemisorption, and thus are suitable foroxygen separation. These complexes may include a transition metal ionand a polydentate ligand. Some examples of suitable complexes are Co orNi or Cu embedded in polyphyrins or oximes, to which axial bases such asnitrogen or sulphur are attached.

Typically, the second layer includes an oxide product of an anodizationprocess. Some examples of such oxides include, but are not limited to,alumina, titania, silica, tin oxide, zirconia, niobium oxide, tungstenoxide, molybdenum oxide, tantalum oxide, aluminosilicate or combinationsof one or more of these. In some embodiments, the second layer mayinclude oxides of alloys metals including aluminum, titanium, tin,zirconium, niobium, tungsten, molybdenum, or tantalum. In an exemplaryembodiment, the second layer includes alumina. Such oxides have theadvantages of thermal, chemical stability, good erosion resistance, andhigh-pressure stability.

FIG. 3 shows a schematic representation of a membrane structureaccording to one embodiment of the present invention. The membranestructure 20 of FIG. 3 includes a first layer 22 having a plurality ofinterconnected pores 23 and a second layer 24 disposed on the firstlayer 12 and having two sublayers 26 and 28 each having a plurality ofunconnected pores 27 and 29. Typically, each of the unconnected pores 27of the sub layer 26 is in fluid communication with at least one of theinterconnected pores of the first layer 12. In one embodiment, thesublayer exposed to the surface such as the sublayer 26 has finer poresizes than a sublayer disposed beneath it such as a sublayer 28. Bytuning the pore dimensions, the properties of the membrane structure maybe controlled to provide performance suitable for any of a number ofapplications. For example, such membrane structures may be utilized ashigh flux membranes with Knudsen selectivity for gases. If the gases donot interact with the membrane surface, membranes prepared using themethod described above could be used to separate gases using a Knudsenmechanism. The advantage of these membranes would be higher fluxes thanfor membranes with thicker active layers.

In another embodiment, the sublayer exposed to the surface such as thesublayer 26 has a coarser pore size than a sublayer disposed beneath itsuch as a sublayer 28. In all the above embodiments, the second layermay include more than two sublayers depending on the requirement of theend use application. The thickness and pore dimensions of each of thelayers are chosen depending on the end use application.

In some embodiments, at least one of the layers includes a catalyticmaterial. For example, by utilizing a catalytic coating or a catalyticlayer within the membrane structure, it is possible to combine membraneseparation with catalytic reaction to achieve high efficiency fluidmixture separation. The catalyzed reaction may be used, for instance, toreduce the concentration of one or more of the reaction products withinthe membrane structure, hence increasing the conversion efficiency.Catalytic materials may also be included in the membrane structure formicroreactor or sensor applications. Some examples of catalysts include,but are not limited to, platinum, palladium, copper, copper oxide,ceria, zinc oxide, alumina, combinations thereof, or alloys thereof.

One skilled in the art would know how to choose a catalyst materialbased on the desired reaction and given working environment, thendispose the desired catalyst into the membrane structure. The catalystsmay be disposed onto the structure by a number of coating techniques.They may be deposited by a physical vapor deposition or by chemicalmeans. Examples of physical vapor deposition include, but are notlimited to, evaporation, e-beam deposition, ion beam deposition, atomiclayer deposition, or a suitable combination of these techniques. Thecatalyst may also be disposed into the membrane structure by means ofchemical vapor deposition. The pores of the membrane structure may alsobe filled with a catalyst by simple capillary filling, or by spraycoating. In such embodiments, the catalyst to be disposed may be takenas a sol, a solution or a gel. In some embodiments, the pore walls ofone or more layers are coated with a catalyst. Alternatively, in someother embodiments, a catalyst layer may be disposed within the membranestructure.

The membrane layers may be functionalized with a suitable functionalgroup to achieve specific functional properties. The functional group,in some embodiments, may be an acid, a basic, an amine, a hydroxyl, acarbonyl, a carboxyl, a mercapto group, a vinyl group, an alkyl, afluoroalkyl, a benzyl, or an acryl group. These functional groups alterthe surface properties of the membrane materials and impart specificproperties to the membranes. For example, the functional groups may beused to change the wettability of the membrane pore surfaces to controlthe flow of fluid through the membrane. Functionalizing the poresurfaces is especially useful for biological or biomedical applicationswhere the membranes desirably be hydrophilic, hydrophobic, lyophobic orlyophilic. The functional groups may be used to control the flow ofspecific chemical or biological species through the membrane. Specificfunctional groups may be used to control the attachment of cells orproteins to the membrane structure. For example, the functional groupsmay also be used to make the membrane structure biocompatible forbiomedical applications. The functional groups may be disposed onto themembrane structure by any known coating technique. In some embodiments,the functional group may be attached to the selected regions of thelayers by exposing the layers to solutions or vapor or ions includingthe desired species. Pretreatment of the layers to enhance the adhesionof the functional groups and masking of regions to be protected duringcoating may be required.

In some embodiments, the membrane structure includes a compositematerial. The composite may include a ceramic-organic or aceramic-ceramic composite. Any ceramic including those listed above maybe used in the composite. The organic material may include a polymer, anoligomer, or a monomer.

The membrane structure of the invention may be useful in a number ofapplications. In some embodiments, the membrane structure is part of aseparation assembly. The membrane structure in certain embodiments ofthe invention may be capable of molecular sieving suitable forpurification of sub quality natural gas, air separation, NO_(x)separation, oxygen separation, or hydrogen recovery from processinggases or feedstock. In one embodiment, the membrane structure of theinvention may be used for separation of hydrogen from nitrogen, argon,carbon dioxide, or methane. In another embodiment, the membranestructure of the invention may be used for separation of volatileorganic components from air streams. In some embodiments the membranestructure is a part of a high temperature gas separation unit. For suchapplications, a suitable metal or a polymer coating may be applied onone or more layers of the membrane structure. Alternatively, a metal ora polymer layer may be used in conjunction with the membrane structure.

FIG. 4 shows a schematic representation of a simple gas separation unit30 according to one embodiment of the invention. The unit 30 includes acompressor 32, a coalescing filter 33 and a pre-heater unit 34 connectedto a membrane separation unit 36. Air under pressure flows first throughthe coalescing filter 33 and then through the pre-heater unit 34 beforereaching the membrane separation unit 36. The coalescing filter may beused to remove oil or water droplets or particulate solids from thefeed. The membrane separation unit includes one or more of membranestructure of the invention configured to remove a desired component fromthe air mixture. The desired component passes through outlet 37, leavingthe waste permeate gases through outlet 38. The membrane separation unitmay include additional heaters or additional filters.

The membrane structure may be used as a liquid-liquid separationassembly such as separation of water from fluid containing organiccomponents. For such applications, the membrane structure may becombined with other porous or non-porous separation layers if needed. Inone embodiment, a separation layer of non-porous cross-linked polyvinylalcohol layer of suitable thickness is used in conjunction with themembrane structure. The pore structure and thickness of each of thelayers may be adjusted depending on the requirement. In someembodiments, the membrane structure may be a membrane structure in aseparation assembly that also includes a reactor component coated on thepore walls to prevent fouling.

In one embodiment, the membrane structure is part of a filtrationassembly. By controlling the pore dimensions of the layers, the membranestructure of the invention may be used for microfiltration to filter outsolid particles with dimensions less than about 10 micrometers, or forultrafiltration to filter out particles with dimensions down to about 50nanometers such as separation of macromolecules and bacteria. Bychoosing the pore dimensions of the layers to very small sizes, it ispossible to use these membrane structures for hyperfiltration to filterout still smaller units such as sugars, monomers, aminoacids, ordissolved ions by reverse osmosis. In one embodiment, the membranestructure is a part of a bio-separation or reaction assembly. The poresize and thickness of the membrane layers are chosen depending on thesizes of the species to be separated. Accordingly in one embodiment, themembrane structure is a filter usable in food, pharmaceutical, andindustrial applications. In another embodiment, the membrane structureis a part of a protein purification unit.

FIG. 5 shows a schematic representation of a simple filter unit 40according to one embodiment of the invention. The unit 40 includes afeed tank 42 used for storing the liquid medium containing the materialto be separated. The circulation of the feed 43 is controlled by thepump 44 that draws the feed 43 through lines 46 and 48 into a membranefilter assembly 50. The membrane filter assembly 50 includes one or moreof the membrane structure of the invention configured to filter out aspecific component from the feed. The desired component ‘filtrate’ 47passes through outlet 49, while the retentate 52 may be removed orreturned to the feed tank 42.

In one embodiment, the membrane structure is part of a reactor assembly,performing similar functions to conventional membranes present inreactors such as filtration and separation. In another embodiment, themembrane structure is capable of reactive separation wherein themembrane structure is a reactor that also separates one of the products.In an exemplary embodiment, the membrane structure is a part of achemical microreactor assembly that generates hydrogen fuel from liquidsources such as ammonia. In such embodiments, suitable hydrogenpermselective catalysts are used in the membrane structure.

In one embodiment, the membrane structure is part of a sensor assembly.In such embodiments, the membrane layers may be functionalized withfunctional groups as discussed above, to incorporate reversible changeswithin the membrane structure. Examples of reversible changes include,but not limited to, chemical reactions such as ionization, oxidation,reduction, hydrogen bonding, metal complexation, isomerization, andcovalent bonding. These changes may be utilized to detect a chemical ora biological species, or to detect change in temperature, pH, ionicstrength, electrical potential, light intensity or light wavelength. Theuse of membrane structures for sensor applications is expected toenhance the performance of detection because of their high surface tovolume ratio.

Another aspect of the invention is to provide a method for preparing amembrane structure. A flow diagram of the method of making a membranestructure is shown in FIG. 6. The method 60 begins with step 62, whereina porous first layer having a plurality of interconnected pores isprovided. In step 64, a electrically conducting coating is deposited onthe porous substrate. In step 66, the electrically conducting layer iscoated with a mask on the surface and anodized through the poroussubstrate in an acidic medium to convert the conducting layer into aporous layer.

In FIG. 7, the schematic of the process steps are shown. To begin with,a porous first layer 70 is provided. Any fabrication technique suitablefor fabricating porous layers may be used to fabricate the first layer.In embodiments where the first layer includes a ceramic, the layer maybe made by a casting process. To start with, a slurry including theceramic powder of the desired material is prepared. The slurry mayinclude a binder and a curing agent. The amount of powder in the slurryis generally adjusted to have the best rheological character. Furtheradditive agents may be mixed into the slurry, such as a dispersing agentfor improving the dispersibility and to prevent rapid settling, and aplaticizer for improving the binding force between the binder and theceramic particles and to lower the risk of cracks. Typically a layer isformed on a substrate by applying the slurry on the substrate. Anytechnique known in the art for preparing layers may be used for formingthe first layer. Non-limiting examples of useful formation techniquesinclude, but are not limited to, spraying, screen printing, ink-jetprinting, casting, wire-bar coating, extrusion coating, gravure coating,roll coating, and combinations thereof. In some exemplary embodiments, acasting technique, such as tape casting, is used. Tape casting provesuseful for making large area thin ceramic sheets with controlledthickness and porosity. The process may include an intermediate a curingprocess to remove organic binders and solvents and a sintering step todensify the layer. Exemplary sintering techniques may involve heating ata specified temperature for a specified duration, or microwaveirradiation, or electron beam irradiation, or UV light exposure, or acombination of those. The porosity, pore size, and pore sizedistribution is controlled by the particle sizes of starting material.It is desirable to start with particles of uniform particle sizes inorder to achieve uniform pore structure with minimal pore sizedistribution. Defect free layers with desired porosity may be obtainedby a precise control of sintering conditions.

In embodiments, where the first layer includes a polymer, any techniqueknown in the art to make porous polymer layers may be used to fabricatethe first layer. For example, a thermoplastic, or a thermoplasticelastomer, or a thermoset polymer may be mixed with a porogen and castinto a thin layer of desired thickness. The layer may be heat treated orexposed to light or any other radiation to convert the layer into aporous layer. When the polymer chosen is a blend of two polymers, it ispossible to control the casting conditions, such as the solvents used,to get a porous layer by phase separation.

In step 64, a coating of electrically conducting material 72 is disposedon the first layer 70. The thickness of the second layer is determinedby the thickness of the deposited conducting coating. Any coatingtechnique known in the art is used for depositing the conducting layer.Some examples of suitable coating techniques include, but are notlimited to exposing the structure to a suspension of particulates, byelectroless deposition or electroplating, or chemical vapor depositionor physical vapor deposition techniques, including atomic layerdeposition.

In step 66, the electrical conducting coating is protected with a mask74 and the conducting layer is anodized in an acid electrolyte throughthe porous first layer. It is known that certain materials such asaluminium, silicon, tin, titanium, zirconium, niobium, tungsten,molybdenum, tantalum, and their alloys form a porous oxide layer whenanodized in an acid medium. The simultaneous formation of oxide layer atthe conducting layer surface and dissolution of the formed oxide intothe acid give rise to a peculiar porous structure including a pluralityof cylindrical pores of uniform size. Typically, a strong acid such as aphosphoric, a sulfuric, or an oxalic acid is used as an electrolyte. Thepore size and the spacing between the pores may be controlled byadjusting the current/voltage during anodic oxidation. The thickness ofthe oxide film formed 76 is controlled by the anodization duration.Anodization may be stopped when the desired thickness of the porousoxide film is grown. After protecting the oxide porous layer, thepassivation layer between the porous oxide layer and the substrate maybe removed by etching to obtain a membrane structure 78. The embodimentsof the method of the invention ensure that each pore formed byanodization has connectivity with the porous support layer.

The controlled anodization of the conducting layer through the poroussubstrate ensures complete connectivity between the pores of thesubstrate and the porous oxide layer. Each of the parallel pores of theoxide film is in fluid communication with at least one of theinterconnected pores of the porous substrate. Membrane structures of theprior art that incorporate anodized oxide layers are generallyfabricated by peeling the metal oxide grown on the substrate from thesubstrate and bonding the oxide layers with a partly sintered particlelayer. The integrated membrane structure of the invention is superior tosuch “sandwiched” structures due to their good mechanical adhesionbetween the layers and the distinct fluid communication between thepores of the two layers.

It is possible to fabricate membrane structures with differentconfigurations by tuning the anodization conditions. For example, themethod of making a membrane structure according to another embodiment ofthe invention is shown as a schematic diagram in FIG. 8. In this case,first a porous first layer 80 having a plurality of interconnected poresis provided. Next, a conducting layer 82 is deposited on the poroussubstrate. The conducting layer 82 is coated with a protective layer 84on the surface and anodized at a particular voltage through the poroussubstrate in an acidic medium to convert the conducting layer into aporous sublayer 86 with a particular pore dimension. Once the desiredthickness of the sublayer 86 is formed, the structure is anodized at areduced anodization voltage to obtain a sublayer 87 with finer pores. Itis possible to introduce any number of sublayers with sequentiallyreducing pore dimensions to fabricate asymmetric membranes by tuning theanodization current or voltage. Finally the passivation layer betweenthe substrate and the metal oxide layer may be removed to obtain anasymmetric membrane structure 88. Alternatively, layers withnon-monotonically graded pore sizes may be fabricated by anodizing fromthe top surface as well as through the porous first layer. In suchembodiments, the layers with the smallest pores may be obtained at thecenter of the anodized layer.

Typically, mesoporous asymmetric inorganic membranes are prepared byforming a mesoporous film on a porous support or by incorporating themesoporous material within the macropores of the support. In suchembodiments, the sol or the gel may clog some of the pores and it isdifficult to control the size and size distribution by any of thesetechniques. Alternatively, asymmetric inorganic membranes are preparedby sandwiching layers with different pore sizes. In such membranestructures, as discussed above, it is difficult to achieve wellconnectivity and good bonding between layers. The method of thisinvention provides a simple and a versatile method to prepare membranestructures with precise pore dimensions and high flux and selectivity.

The following example serves to illustrate the features and advantagesoffered by the present invention, and not intended to limit theinvention thereto.

EXAMPLE

The following examples describe the preparation method for makinganodized alumina membrane structures.

Example 1 Method for Fabricating a Membrane Structure of AnodizedAlumina on Porous Support Layer

A continuous aluminum coating of 1 micrometer to 2 micrometers thick isdeposited on a porous alumina support. The aluminum surface is maskedwith nail polish to protect it. The aluminum is then anodized throughthe porous support in oxalic acid at voltages greater than 20 V ensuringthat the pores in the anodized alumina layer that forms are connected tothe substrate porosity. The anodized alumina pore diameter is determinedby the anodization voltage. The anodized alumina layer thickness isdetermined by anodization time. Once the anodized alumina layer is ofthe desired thickness, anodization is stopped and the balance aluminumis etched from the surface using a copper chloride solution. Finally abarrier oxide layer is etched away using 5 wt % phosphoric acid toreveal the top surface of the porous alumina layer.

Example 2 Method for Fabricating a Membrane Structure of AsymmetricAnodized Alumina on Porous Support Layer

A continuous aluminum coating of 1 micrometer to 2 micrometers isdeposited on a porous alumina support. The aluminum surface is maskedwith nail polish to protect it. The aluminum is then anodized throughthe porous support in oxalic acid at voltages greater than 20 V, asdescribed above. Once the anodized alumina layer is of the desiredthickness, anodization voltage is decreased to get a porous sublayerwith finer pores. Multiple alumina sublayers with decreasing pore sizemay be fabricated by sequential reduction in voltage. Any remainingaluminum is etched from the surface using a copper chloride solution.Finally a barrier oxide layer is etched away using 5 wt % phosphoricacid to reveal the top surface of the porous alumina layer.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A membrane structure comprising: a first layer having a plurality ofinterconnected pores; and a second layer disposed on the first layer,the second layer having a plurality of unconnected pores, wherein eachof the unconnected pores is in fluid communication with at least one ofthe interconnected pores of the first layer.
 2. The membrane structureof claim 1, wherein the first layer comprises a material selected fromthe group consisting of a ceramic and a polymer.
 3. The membranestructure of claim 1, wherein the first layer has a porosity greaterthan about 1%.
 4. The membrane structure of claim 3, wherein the firstlayer has a porosity in a range from about 20% to about 70%.
 5. Themembrane structure of claim 4, wherein the first layer has a porosity ina range from about 30% to about 50%.
 6. The membrane structure of claim1, wherein the second layer comprises an oxide of a material selectedfrom the group consisting of aluminum, titanium, silicon, zirconium,niobium, tungsten, molybdenum, tantalum, combinations thereof, andalloys thereof.
 7. The membrane structure of claim 1, wherein the secondlayer comprises a material selected from the group consisting ofalumina, zirconia, and titania.
 8. The membrane structure of claim 6,wherein the second layer comprises alumina.
 9. The membrane structure ofclaim 1, wherein the second layer has a thickness less than about 10micrometers.
 10. The membrane structure of claim 1, wherein the secondlayer has a thickness in the range from about 10 nanometers to about 500nanometers.
 11. The membrane structure of claim 10, wherein the secondlayer has a thickness in the range from about 10 nanometers to about 100nanometers.
 12. The membrane structure of claim 1, wherein the secondlayer has a median pore size of less than about 1 micrometer.
 13. Themembrane structure of claim 1, wherein the second layer has a medianpore size in the range from about 1 nanometer to about 500 nanometers.14. The membrane structure of claim 13, wherein the second layer has amedian pore size in the range from about 1 nanometer to about 40nanometers.
 15. The membrane structure of claim 1, wherein the secondlayer comprises a fine pore sublayer disposed on a coarse pore sublayer,the coarse pore sublayer disposed on the first layer.
 16. The membranestructure of claim 1, wherein the first layer comprises more than onesublayer.
 17. The membrane structure of claim 1, wherein at least one ofthe layers comprises a catalytic material.
 18. The membrane structure ofclaim 1, wherein at least one of the layers comprises a functionalgroup.
 19. The structure of claim 18, wherein the functional group is atleast one selected from the group consisting of an amine, a carboxyl, amercapto, a carbonyl, a hydroxyl, a vinyl, an alkyl, a benzyl, afluoroalkyl, and an acryl group.
 20. The membrane structure of claim 1,wherein the membrane structure comprises a metal.
 21. The membranestructure of claim 20, wherein the metal comprises a transition metal.22. The membrane structure of claim 20, wherein the metal comprises atleast one selected from the group consisting of a platinum group metal,iron, nickel, cobalt, copper, combinations thereof, and alloys thereof.23. The membrane structure of claim 1, wherein the membrane structurecomprises an organic material.
 24. The membrane structure of claim 23,wherein the organic material comprises a polymer.
 25. The membranestructure of claim 24, wherein the polymer is one selected from thegroup consisting of polysulphones, polyamides, cross-linked polyimides,polyether ketones, polyetherimides, silicone rubber, nitrile rubber,neoprene rubber, silicone, polycarbonate, polyarylene, polyphenyleneether, polyolefin elastomer, polybutadiene, vinyl polymers, combinationsthereof, and block copolymers thereof.
 26. A separation assemblycomprising the membrane structure of claim
 1. 27. A gas separationassembly comprising the membrane structure of claim
 1. 28. A hightemperature gas separation assembly comprising the membrane structure ofclaim
 1. 29. A filtration assembly comprising the membrane structure ofclaim
 1. 30. A reactor assembly comprising the membrane structure ofclaim
 1. 31. A sensor assembly comprising the membrane structure ofclaim
 1. 32. A membrane structure comprising: a first layer having aplurality of interconnected pores; and a second layer disposed on thefirst layer, the second layer comprising an anodized alumina having aplurality of unconnected pores, wherein each of the unconnected pores isin fluid communication with at least one of the interconnected pores ofthe first layer.
 33. A membrane structure comprising: a first layerhaving a plurality of interconnected pores; and a second sublayerdisposed on the first layer, the second sublayer comprising an anodizedalumina having a plurality of unconnected coarse pores; a third sublayerdisposed on the second sublayer, the third sublayer comprising ananodized alumina having a plurality of unconnected fine pores, whereineach of the unconnected fine pores of third sublayer is in fluidcommunication with at least one of the interconnected pores of the firstlayer.
 34. A method comprising: providing a first layer having aplurality of interconnected pores; and disposing a second layer on thefirst layer.
 35. The method of claim 34, wherein disposing a secondlayer comprises depositing a conducting layer on the first layer; andanodizing the conducting layer to convert the conducting layer intoporous layer.